This invention generally relates to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling transmission power within TDD communication systems.
FIG. 1 depicts a wireless spread spectrum time division duplex (TDD) communication system. The system has a plurality of base stations 301-307. Each base station 301 communicates with user equipments (UEs) 321-323 in its operating area. Communications transmitted from a base station 301 to a UE 321 are referred to as downlink communications and communications transmitted from a UE 321 to a base station 301 are referred to as uplink communications.
In addition to communicating over different frequency spectrums, spread spectrum TDD systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). Also, to more efficiently use the spread spectrum, TDD systems as illustrated in FIG. 2 use repeating frames 34 divided into a number of time slots 361-36n, such as fifteen time slots. In such systems, a communication is sent in selected time slots 361-36n using selected codes. Accordingly, one frame 34 is capable of carrying multiple communications distinguished by both time slot 361-36n and code. The combination of a single code in a single time slot is referred to as a resource unit. Based on the bandwidth required to support a communication, one or multiple resource units are assigned to that communication.
Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. When a UE 321 or base station 301 is receiving a specific communication, all the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. However, reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used.
One approach to control transmission power levels is open loop power control. In open loop power control, typically a base station 301 transmits to a UE 321 a reference downlink communication and the transmission power level of that communication. The UE 321 receives the reference communication and measures its received power level. By subtracting the received power level from the transmission power level, a pathloss for the reference communication is determined. To determine a transmission power level for the uplink, the downlink pathloss is added to a desired received power level at the base station 301. The UE's transmission power level is set to the determined uplink transmission power level.
Another approach to control transmission power level is closed loop power control. In closed loop power control, typically the base station 301 determines the signal to interference ratio (SIR) of a communication received from the UE 321. The determined SIR is compared to a target SIR (SIRTARGET). Based on the comparison, the base station 301 transmits a power command, bTPC. After receiving the power command, the UE 321 increases or decreases its transmission power level based on the received power command.
Both closed loop and open loop power control have disadvantages. Under certain conditions, the performance of closed loop systems degrades. For instance, if communications sent between a UE and a base station are in a highly dynamic environment, such as due to the UE moving, such systems may not be able to adapt fast enough to compensate for the changes. The update rate of closed loop power control in TDD is 100 cycles per second which is not sufficient for fast fading channels. Open loop power control is sensitive to uncertainties in the uplink and downlink gain chains and interference levels.
One approach to combining closed loop and open loop power control was proposed by the Association of Radio Industries and Business (ARIB) and uses Equations 1, 2, and 3.TUE=PBS(n)+L  Equation 1PBS(n)=PBS(n−1)+bTPCΔTPC  Equation 2                              b          TPC                =                  {                                                                                          1                    ⁢                                          :                                                                                                                                                              -                      1                                        ⁢                                          :                                                                                            ⁢                                                                                                      if                      ⁢                                                                                           ⁢                                              SIR                        BS                                                              <                                          SIR                      TARGET                                                                                                                                                              if                      ⁢                                                                                           ⁢                                              SIR                        BS                                                              >                                          SIR                      TARGET                                                                                                                              Equation        ⁢                                   ⁢        3            
TUE is the determined transmission power level of the UE 321. L is the estimated downlink pathloss. PBS(n) is the desired received power level of the base station 301 as adjusted by Equation 2. For each received power command, bTPC, the desired received power level is increased or decreased by ΔTPC. ΔTPC is typically one decibel (dB). The power command, bTPC, is one, when the SIR of the UE's uplink communication as measured at the base station 30, SIRBS, is less than a target SIR, SIRTARGET. Conversely, the power command is minus one, when SIRBS is larger than SIRTARGET.
Under certain conditions, the performance of these systems degrades. For instance, if communications sent between a UE 32 and a base station 30 are in a highly dynamic environment, such as due to the UE 32 moving, the path loss estimate for open loop severely degrades the overall system's performance. Accordingly, there is a need for alternate approaches to maintain signal quality and low transmission power levels for all environments and scenarios.